In Patent Document 1, an allowable discharge electric power is calculated for use in temporarily relaxing limitation on discharge. Specifically, the current value (maximum dischargeable current value) of a battery found when the discharge electric power of the battery is increased until the voltage value of the battery reaches a lower limit voltage value is calculated on the basis of the present current value, the present voltage value and the present internal resistance value of the battery. The allowable discharge electric power (maximum dischargeable electric power) is obtained by multiplying the maximum dischargeable current value by the lower limit voltage value.